Swimming Pool (C-53) · Lesson 3 of 8

Suction Outlets and Entrapment: Two Drains and the Velocity Limits

"Suction Outlets and Entrapment: Two Drains and the Velocity Limits" is lesson 3 of 8 in the Swimming Pool (C-53) Learning Hub. It opens with "The Two-Outlet Rule" and works through 6 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. The Two-Outlet Rule
  2. Private Pools and Covers That Need Tools
  3. The Velocity Limits
  4. Velocity Math
  5. Single Outlets and the Printed Anti-Entrapment Systems
  6. Equalizer Lines and Solar Suction Distance

The worked example in the course applies this lesson to a realistic job in 6 steps, from "Check the layout" through "Return piping".

What you'll be able to do

  • Explain the main idea of "Suction Outlets and Entrapment: Two Drains and the Velocity Limits" in plain language you can use on the exam.
  • Apply this step with confidence: "The Velocity Limits".
  • Apply this step with confidence: "Velocity Math".
  • Apply this step with confidence: "Single Outlets and the Printed Anti-Entrapment Systems".
  • Apply this step with confidence: "Equalizer Lines and Solar Suction Distance".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Suction Outlets: Two Drains, Three Feet, Six ft/sChecking branch pipe size for 93.5 gpmPOOL FLOOR (plan view)ABat least 3 ft apart (4 ft shown)T fittingPUMP46.75 gpm46.75 gpmNormal: each branch carries half of 93.5 gpmOne blocked: open branch carries all 93.5 gpmVELOCITY = gpm / (448.831 x A)2.0 in. ID branch A = 0.02182 sq ftNormal 4.77 ft/s (limit 3) FAILSBlocked 9.55 ft/s (limit 6) FAILS3.0 in. ID branch A = 0.04909 sq ftNormal 2.12 ft/s (limit 3) PASSESBlocked 4.24 ft/s (limit 6) PASSESSmallest passing ID: 93.5 / (448.831 x 6)A = 0.03472 sq ft = 2.52 in. inside diameterReturn piping at 8 ft/s needs 2.19 in. IDTwo balanced outlets, 3 ft apart, branch ID at least 2.52 in.Limits between outlets: 3 ft/s normal, 6 ft/s with one outlet blockedSuction pipe 6 ft/s, return pipe 8 ft/s (§3125B.1)2025 CBC §§3125B.1, 3137B.1, 3162B; HSC §115928; 2025 CPC Useful TablesCC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: The Two-Outlet Rule

A single blocked main drain can hold a swimmer under water, so California prints a layout rule. 2025 CBC §3137B.1(1) says each pump on a pool system shall be connected to at least two suction outlets that are hydraulically balanced, symmetrically plumbed through one or more "T" fittings, and separated by at least 3 feet in any dimension. §3162B(5) requires the same of every public pool built on or after January 1, 2010.

Step 2: Private Pools and Covers That Need Tools

For a NEW private pool or spa permit, HSC §115928(a)(2), printed in 2025 CRC Appendix CI, requires either two such outlets per pump or a design that uses alternatives such as skimmers or perimeter overflow systems to conduct water to the pump. Every suction fitting must meet ANSI/APSP-16 (2025 CBC §3137B.1(2)), and under §115928(b) the covers are anti-entrapment grates that cannot be removed except with tools. A permit to remodel or modify an existing pool, toddler pool or spa requires its suction outlets to be upgraded to anti-entrapment grates (§115928.5).

The remaining 4 steps, the worked example and the quiz are in the course. This is a preview. The full lesson, with the step-by-step explanations, a worked example and a quick-check quiz, is inside the classroom.

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